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The energetic constraints on the Zonal Mean Atmospheric Circulation: Why is ITCZ in the North? And Why Do GCMs Have Double ITCZ?

The Northern Hemispheric maximum of the tropical rainfall is usually thought to be caused by properties of tropical landmasses, such as the size and shape of continents. However, a variety of recent studies have established that conditions even well outside the tropics also affect tropical circulation and rainfall. This new understanding has reopened the question of why tropical rainfall peaks in the Northern Hemisphere (NH). In this talk, I will demonstrate that the meridional overturning circulation of the ocean is instead the cause of the peak of zonal mean rainfall north of the equator, by causing a cross-equatorial ocean heat transport that heats the NH atmosphere more than the SH. We use satellite observations of the Earth’s energy budget and atmospheric reanalyses to study tropical rainfall using a global energetic framework. Global climate model simulations even without continents show a pronounced peak of precipitation in the NH when the observed oceanic heat transport is added.

Using the same energetic framework, we found that a large fraction of the double ITCZ problem can be attributed to a lack of low cloud coverage over Southern Ocean, which produces an excessively warm SH and an ITCZ that maximizes too much in the southern tropics.